1. Field of the Invention
This invention relates to ultrasonic transducers of the type employed in ultrasonic flowmeters.
2. The Prior Art
Transducers of this type employ one or more piezoelectric elements. When used in the pulse mode the piezoelectric element is excited by short duration voltage pulses, typically one or two microseconds in duration. Upon application of a voltage pulse a piezoelectric element undergoes dimensional changes. These dimensional changes produce acoustic pressure waves in the contiguous medium, and this acoustic wave is utilized in ultrasonic flowmeters to gauge the speed of fluid flow within the meter.
When a piezoelectric element is excited by a short duration voltage pulse the dimensional changes which the element undergoes should ideally disappear as soon as the voltage pulse is removed. However, piezoelectric elements have elastic properties. The elements therefore oscillate in a dampened sinusoidal manner following excitation.
To dampen the oscillations in a piezoelectric element, it is a known practice to secure to the rearward face of the element a dampening member. The members are formed of material which dissipates energy and prevents acoustic energy from being reflected back from the element into the crystal. It has been a common practice to bond the dampening element to the rearward face of a piezoelectric crystal utilizing cement. This method of bonding is usually suitable for temperatures over a range of approximately 0.degree. to 100.degree. C. However, at cryogenic temperatures, such as that of liquid nitrogen (-196.degree. C.), the difference in thermal expansion of the piezoelectric element and the dampening member may cause the cement bond to fail. If this bond fails, then dampening provided by the dampening element is substantially removed. When dampening is lost the ultrasonic device may itself fail.
This invention overcomes the problems of the prior art by providing a compact transducer including a piezoelectric crystal and a dampening member and providing means of securely maintaining the dampening member in contact with the piezoelectric element even at cryogenic temperatures.